A. Field of the Invention
The present invention relates to electronic data storage devices. More particularly, the invention relates to a modular disk memory apparatus which employs a plurality of small disk drives, control electronics, and software functionally interconnected in a manner which optimizes sustained rates of data transfer per unit volume of the apparatus.
B. Description of Background Art
A substantial variety of applications exist for high capacity electronic digital storage devices. While memory capacities of one megabyte (1 million 8-bit words or bytes) were considered to be relatively large only a few years ago, many small personal computers (PC's) now have memory capacities of two gigabytes (two thousand-million bytes) or larger.
Most general purpose work stations, PC's and network computers use one or more hard disk drives for storing large blocks of data. Hard disks are magnetic memory devices which each have a data storage capacity of up to about six gigabytes, and a relatively high data transfer rate. A typical hard disk drive consists essentially of a thin, relatively inflexible disk rotating at high speed, having a diameter of about 5 inches and coated with a ferromagnetic material having tiny domains magnetizable in either of two directions to record digital data as a series of "ones" or zeros" in circular-tracks on the disk.
For certain applications requiring large blocks of data to be transferred at high rates, existing PC-type hard disk memories are inadequate. One solution to the problem of providing large data storage capacities is to employ a plurality of functionally interconnected, small, relatively inexpensive disk drives. To allow for the possibility that, on occasion, one or more of such drives will malfunction, temporarily experience defects in one or more data storage tracks, or fail entirely, the architecture of large capacity disk memory storage units often incorporates one or more levels of redundancy. Thus, such high capacity data storage units usually employ at least one dedicated disk drive that performs a parity checking or back-up data storage function. The dedicated disk drive provides a capability of reconstructing data which may be lost because of a defect or malfunction in one of the disk drives. Because of this redundancy function, a data storage device of this type is sometimes referred to as a RAID (Redundant Array of Inexpensive Disks).
A variety of data storage devices employing a plurality of disk drives, some of such devices configuring the disk drives in a RAID type of array, are disclosed in the following U.S. patents:
Crater et al., U.S. Pat. No. 5,146,580, Sep. 8, 1992, Redundancy Accumulator For Disk Drive Array Memory: Discloses a data storage subsystem that uses a large plurality of small form-factor disk drives (typically 51/4 inch) to implement an inexpensive, high performance, high reliability disk drive memory that emulates the format and capability of large form-factor disk drives. The data transmitted by the associated computer system is used to generate redundancy information which is written with the data across N+M disk drives in a redundancy group in the data storage subsystem.
Hotle, U.S. Pat. No. 5,218,689, Jun. 8, 1993, Single Disk Emulation Interface For An Array Of Asynchronously Operating Disk Drives: Discloses a multiple disk drive array storage device that emulates the operation of a single disk drive. The array storage device includes a large buffer memory and a plurality of asynchronously-operating disk drives (typically 14-inch diameter).
Rudeseal et al., U.S. Pat. No. 5,239,659, Aug. 24, 1993, Phantom Duplex Copy Group Apparatus For A Disk Drive Array Data Storage Subsystem: Discloses an apparatus that uses a disk drive array to store data records for an associated host processor. This disk drive array emulates the operation of a large form factor disk drive by using a plurality of interconnected small form factor disk drives. These small form factor disk drives are configured into at least two redundancy groups, each of which contains n+m disk drives for storing data records and redundancy information thereof.
Fisher et al., U.S. Pat. No. 5,398,158, Mar. 14, 1995, Multiple Disk Drive Module With Standard Form Factor: Discloses a disk drive module which externally has the physical and electrical appearance of a single standard 5.25 inch disk drive, but which internally contains five 2.5 inch disk drives and the electronics for operating them as resilient disk array. Each of the disk drives is mounted on a separate printed circuit board which extends substantially the full length of the module.
Hao et al., U.S. Pat. No. 5,412,661, May 2, 1995, Two-Dimensional Disk Array: Discloses a data storage system architecture having an array of small data storage disks, organized into logical rows and columns, with each disk coupled to two disk controllers via two independent controller-disk interconnects.
Nunnelley et al., U.S. Pat. No. 5,423,046, Jun. 6, 1995, High Capacity Data Storage System Using Disk Array: Discloses a data storage and retrieval system which has extremely high capacity. The system includes a large array of small disk files, and three storage managers for controlling the allocation of data to the array, access to data, and the power status of disk files within the array.
Blair et al., U.S. Pat. No. 5,539,660, Jul. 23, 1996, Multi-Channel Common-Pool Distributed Data Storage And Retrieval System: Discloses a system that includes a plurality of disk arrays for storing, sequentially, contiguous segments of a data block. In the case of movies, each segment includes, for example, a few seconds of each relevant movie. An electronic commutator sequentially connects the disk arrays to a corresponding plurality of access channels.
Baba, U.S. Pat. No. 5,544,339, Aug. 6, 1996, Array Of Disk Drives With Redundant Channels: Discloses an array of disk drives that stores information which is accessed through multiple channels by a host computer. Different channels are coupled to different sequences of disk drives. Different disk drives can be accessed simultaneously through different channels, enabling high data transfer rates.
Matsumoto et al., U.S. Pat. No. 5,517,632, May 14, 1996, Redundant Array Of Disks With Improved Storage And Recovery Speed: Discloses a redundant array of disks in which the disks are divided into areas of different sizes, so that small amounts of data can be stored in an area of an appropriate size on a single disk, instead of being spread over multiple disks. A usage status table indicates which areas are in use.
The above-listed references are directed generally to architectures which utilize a plurality of disk drive memories to achieve high data storage capacities, with the goal of some of the devices to also possess high data transfer rates. However, existing large capacity data storage units, particularly those employing a RAID architecture, have certain limitations. In particular, most such devices are capable of transferring data at high rates only in short, interrupted, bursts. Also, presently existing data storage units are relatively large. These limitations are disadvantageous for a variety of applications, including high speed data acquisition, high performance work stations or networking, digital video and movies, weather prediction, oil prospecting and seismic measurements, and satellite communications. The present invention was conceived of to provide a modular data storage apparatus having a high modularly selectable data storage capacity, very high sustained, absolute data transfer rates, and data transfer rates that are maximized per unit volume of the apparatus.